Capillary arc plasma source for and method of spectrochemical analysis

ABSTRACT

A wall-stabilized electric arc within a capillary tube, supported by a gas flowing lengthwise through the tube is used to excite a material for spectroscopic analysis. A material to be analyzed, in gaseous or finely comminuted form, is fed into the arc near one end and carried through it by the working gas. Emission or absorption of radiation is observed endwise of the tube.

United States Patent Dahlquist et al.

1341 CAPILLARY ARC PLASMASOURCE IJRAND METHOD OF SPECTROCHEMICAL ANALYSIS Inventors: Ralph L. Dahlquist; James L. Jones,

both of Santa Barbara, Calif.

Assignee: Applied Research Labortories, lnc.,

Sunland, Calif.

Filed: June 17, 1968 Appl. No.: 737,633

US. Cl ..356/86, 313/231 1m. 01 ..G0lj 3/30 Field of Search ..356/86; 315/111; 313/231 [56] References Cited UNITED STATES PATENTS 3,458,258 7/1969 Krugers ..356/86 [451 Aug. 22, 1972 3,512,030 5/1970 Levy ..3l5/l 11 Primary Examiner- Ronald L. Wibert Assistant Examiner-Y. P. McGraw Attorney-Hoffman Stone [57] ABSTRACT 1 absorption of radiation is observed endwise of the tube.

1 2 Figures CAPILLARY ARC PLASMA SOURCE FOR AND METHOD OF SPECTROCHEMICAL ANALYSIS BRIEF SUMMARY This invention relates to novel methods of and apparatus for spectrochemical analysis by optical emission, or atomic absorption, or both, and, more particularly, to novel methods of an apparatus for producing an electric arc, introducing a material to be analyzed into the arc, and observing effects that occur in it.

Spectrochemical analysis by optical emission and by atomic absorption are well known and widely used techniques. Generally in these techniques, a material to be analyzed is heated to a relatively high temperature to evaporate, dissociate and excite the atoms of the material..ln optical emission analysis, light emitted by the material is analyzed; in atomic absorption analysis, light from a separate source is directed through the excited material, and the attenuation is measured.

Many different arrangements have been used and proposed heretofore for exciting materials to be analyzed. All, however, are subject to various inherent disadvantages or limitations.

Accordingly, one important object of the invention is to provide means for exciting a material for analysis by optical emission or atomic absorption, which avoids or overcomes many of the limitations and difficulties of the prior art arrangements, is of simple and inexpensive construction, easy to use, and reliable and long lived in service.

Briefly, the invention contemplates the use of an electric arc plasma for exciting a material to be analyzed. The are is wall-stabilized within a capillary tube, and is sustained by a working gas flowing through the tube. The material to be analyzed, in gaseous or nebulized form, is carried through the are by the working gas. Because of the wall-stabilized effect, the arc operates at a very high power density, and is highly stable in position and shape.

The direction of observation is preferably along the length of the arc to insure optimum signal detection regardless of the location in the are where the signals originate or occur at strength. Self-absorption effects are small as compared to other, prior arrangements. The device is capable of operating continuously, quietly and inexpensively for long periods of time, is convenient and easy to use, rugged and long lasting in service, and is advantageous in many other respects, as will become apparent hereinafter.

DETAILED DESCRIPTION Representative embodiments of the invention will now be described in detail in connection with the accompanying drawings, wherein:

FIG. 1 is a cross-sectional view, partly in schematic form, of an electric arc plasma source according to a first embodiment of the invention; and

FIG. 2 is a cross-sectional view, partly in schematic form, of an electric arc plasma source according to another embodiment of the invention, as arranged to accommodate analysis by atomic absorption.

A plasma source according to the first embodiment as shown in FIG. 1 is arranged for analysis by optical emission. It includes a generally cylindrical vessel 10 having a small opening 12 at one end, a window 14 at the end opposite from the opening 12, a gas inlet port 16 extending through the side wall of the vessel near the window 14, and a cooling arrangement such as the annular passage 18 for circulating a liquid to cool the wall of the opening 12. The material around the opening 12 is electrically and thermally conductive. It serves as the anode for the arc.

A disc, or plate 20 of a refractory, insulating material such as, for example, boron nitride, is fitted in abutting relationship upon the end of the vessel 10 across the anode 12, and has a passage, or bore 22 extending through it. The bore 22 constitutes the introduction chamber for the material to be analyzed. A feed tube 24 extends laterally from the chamber 22 to the exterior of the plate 20.

The capillary tube 26 is formed in a thermally conductive, dynamically cooled, cap-like body 28 fitted to the plate 20 on the opposite side of the plate from the vessel 10. The tube 26, the introduction chamber 22, and the anode 12 are all coaxially aligned.

The term capillary as used in this application is not intended to be limited to the connotation of hair-like thinness, as in other arts, but to include tubes of substantially larger dimensions such as of one-eighth or even one-quarter inch diameter, depending on the atmospheric pressure in the arc. The term capillary is intended to denote that the tube is of small enough diameter to keep the arc wall-stabilized. The wall of the tube cools the outer portion of the gas flowing through it so that the outer portion of the gas is not heated enough to make it electrically conductive. Only a small part of the gas along the central axis of the tube carries the'discharge. The heat losses to the wall of the tube are high, and the arc must be operated at a very high power density to sustain it. Because of this effect, the position and shape of the are are stable, and signals of high intensity are produced.

A cathode 30 of electron emissive material, which may be, for example, a wire of 99 percent tungsten/l percent thorium alloy, is mounted on an insulating support 32 in a bore 34 extending laterally from the tube 26 at the end opposite from the plate 20. The tip of the cathode 30 is recessed from the tube 26. An exhaust port 36 vents the bore 34 and the tube 26 to atmosphere.

In operation, a working gas such as argon, typically at normal atmospheric pressure, is introduced both through the port 16 adjacent to the window 14 and through the feed tube 24. Air is rapidly flushed from the interior of the device, and the atmosphere within it is soon constituted practically entirely of the working gas. The are is initiated. This may be done by the application of a momentary high voltage spark. When the body member 3 is of an electrically conductive material, the arc first strikes from the anode 12 to the nearest point in the tube 26, and from the cathode 30 to the point nearest to it of the body member 28. The arc persists in this condition for a short period of time, during which the tip of the cathode 30 becomes heated. When the tip of the cathode 30 becomes sufficiently hot, the arc transfers to a free condition between the anode 12 and the cathodes 30, and becomes wall-stabilized. This will occur automatically under most conditions if the tube 26 is not long, but if it does not, the transfer may be facilitated by briefly increasing the flow of working gas, as may be necessary in most cases when the tube 26 is longer than about one inch.

through the feed tube 24 entrained with the working I gas. The light to be analyzed is observed through the window 14, looking endwise into the tube 26 through the anode 12. The device operates at substantially atmospheric pressure, and requires only relatively small amounts of working gas, typically about 3 to 5 cubic feet per hour.

The actual dimensions of the various parts of the apparatus are not limiting factors in the practice of the invention, but may be varied within wide limits in accordance with the designers choice. In an actual embodiment, which has been successfully operated for many hours, the capillary tube 26 is about one-eighth inch in diameter and three-quarters inch long. The introduction chamber is about one-quarter inch in diameter and five-sixteenths inch long. The anode opening 12 is about one-eighth inch in length and in diameter. The window 14 is about 1% inch in diameter presently believed that some spacing will be found desirable in almost all cases because fine particles of solids if present in the plasma will tend to diffuse upstream against the gas flow. The diameter of the win- -dow 14 is selected in view of the optical input to the spectrometer to be used for viewing the arc. It is preferably fairly large to maximize the acceptance angle, and, thereby, the amount of light from the are that is fed to the spectrometer. The cathode 30 is preferably recessed from the tube 26 to minimize the effects of light emitted by it.

Typical operating conditions in the embodiment just described are as follows:

Gas entering through the tube 24 Argon, at 3 cu. ft. per hr. Gas entering through the port 16 near the window Argon, at 1 cu. ft. per hr. Arc current 5 amperes initiator high voltage spark source Material under analysis one milligram per minute,

sub-micron particles in dry aerosol For analysis by atomic absorption, the source is modified as illustrated inFlG. 2 by the addition of an auxiliary cylindrical vessel 40 opening from the end of the tube 26' opposite from the anode 12. A window 42 at the end of the auxiliary vessel 40 enables the passage of light or other radiation from any. selected source (not shown) into the tube 26 from the cathodic end. A port 44 is provided in the side wall of the auxiliary vessel for admitting working gas.

With this embodiment of the invention, the arc may be viewed from either end. When viewed from the cathodic end, significant amounts of radiations from the cathode 30 may be observed, and may at times interfere with the analysis. For this reason, it is presently preferred to make all spectrometric measurements from the anodic end.

The light source used for analysis by atomic absorption may be mounted within the auxiliary vessel 40, if

desired, and if it is mounted outside, it may be ad-- vantageous in some cases to make the window 42 in the, form of a lens to concentrate the light upon the capilla-.

The working gas admitted through the port 16 in thefirst vessel, and also through the port 44 in the second one, serves to keep the windows 14 and 42,respectively, clean and clear of the plasma, which might otherwise be drawn into the vessels 10 and 40 due to reduction of pressure in them during operation.

The device of the invention successfully overcomes most of the disadvantages and limitations of the prior art arrangements. Interfering spectra, either continuous or discrete, are limited to those produced by substances intentionally introduced. By judicious selection of the working gas, interference between radiation produced by it and the radiation it is desired to measure may be eliminated. Consumption of inert gases (the working gas) is less by an order of magnitude compared to plasma devices of the prior art thatoperate at atmospheric pressure. Use in the far ultra violet region is limited only by the type of window material used and the transparency'of the working gas. Ifthe window 14 is of lithium fluoride, for example, radiation may be detected at wavelengths as short as about 1,100 angstroms. The device may be readily coupled directly to a vacuum spectrometer. Power supply requirements are relatively small and inexpensive, especially as compared to induction plasmas of the prior art and devices of the interrupted discharge type. Erosion and the consequent repetitive maintenance of thecathode 30 are almost completely eliminated, and greatly reduced compared to the erosion and maintenance requirements of direct current plasma jetsof the prior art.

The tube 26 may be made long enough to provide highly sensitive atomic absorption measurements. In addition, operating parameters such as the arc current and the flow of working gas may be optimized independently of and without affecting the process or means by which the material being analyzed is obtained. Also, the material being analyzed need not be electrically conductive, as in the case of conventional arc and spark discharge sources. Any material can be fed into the are provided only that it can be put in the form of a gas or nebulized. I

If desired, aperture 12 or 12' may be partly insulated, or a separate anode (not shown) may be used to localize the anodic end of the arc, thus further to enhance the positional stability of the arc.

It is presently believed that the direction of flow of the working gas is not critical in the practice of the invention, and that generally similar analytical results may be achieved if the flow is directed from the cathode toward the anode. This would, of course, require a relocation of the introduction chamber, because it is important to have the material under analysis traverse a substantial length of the arc.

The term light as used herein is intended to include not only visible light, but also radiation in adjacent parts of the spectrum such as ultra-violet radiation and such other as is useful in spectrometric analytical work of the optical emission and atomic absorption kinds.

What is claimed is:

1. A capillary arc plasma device comprising:

a. means defining a straight tubular passageway,

b. means defining an anode at one end of said 

1. A capillary arc plasma device comprising: a. means defining a straight tubular passageway, b. means defining an anode at one end of said passageway, c. means electrically insulating said anode from a substantial lengthwise part of the inwardly facing surface that bounds said passageway, and d. a cathode adjacent to the end of said passageway opposite from said anode and radially offset from said passageway, said cathode being shielded from the anode end of the passageway to minimize transmission of light from the cathode through the passageway and enable end-on spectrometric observation of an arc in the passageway without excessive background. 